1. Field of the Invention
This invention relates in general to novel copolymeric materials suitable for use as a release agent or as a component in a release composition, and to methods for their preparation. It more particularly relates to coated substrates having a surface coated therewith such as release liners and pressure-sensitive adhesive tapes.
2. Description of the Prior Art
Pressure-sensitive adhesive tapes are generally manufactured and sold with the tape wound upon itself in convolutions to form a roll of some suitable length of tape. Consequently, when it is necessary to use the tape, it must be possible to unwind the desired length from the roll without excessive force or delamination of the backing, offsetting of the adhesive, or the like, regardless of the time or conditions under which the tape has remained in roll form prior to use. For these reasons, a coating known as a release coat or back size is generally provided on the back side of the tape backing member, i.e., the side opposite that on which the adhesive mass is applied. Such a coat, compared to an uncoated backing member, as is its objective, offers relatively low adhesion to the adhesive mass.
Although various release agents and compositions thereof have been developed over the years, none of them, to my knowledge, accomplish all the objects desired by their use. With some release agents, the release characteristics diminish with time, and particularly at high temperatures because of some chemical or physical change in the release material per se. Others interact with the adhesive mass, sometimes merely physical, so that adhesion to various substrates to which the tape is applied is adversely affected.
The extension of the pressure-sensitive concept beyond its early more limited usage has spurred the need for, in certain applications, more aggressive adhesives. As a result there have been rather simultaneous demands for improved release coatings to permit suitable removal of the adhesive from the backing member at the time of application.
Whether a material will be suitable or not as a release agent, in particular for pressure-sensitive adhesive tapes, depends upon a number of factors. The lower the interfacial tension between the adhesive mass and the material used as a release coating, naturally the better release provided; however, low interfacial tension alone is not enough. The material, to be useful as a release coat, should also be of a suitable cohesive strength and possess good adhesion with the backing member.
In the early 1950's, silicones were introduced as coatings for paper substrates to provide release surfaces for tacky materials. One use for these coated substrates was as backing members for pressure-sensitive adhesives. The silicones are a class of synthetic materials which are composed primarily of alternating silicon and oxygen atoms in the polymeric backbone. A variety of organic groups can be attached to the silicon atoms to make up the polymers that are referred to as organopolysiloxanes. The simplest polysiloxane, polydimethylsiloxane, is one in which methyl groups are attached to the silicon atoms. The physical nature of these polymers, e.g., whether resin, rubber, or oil, depends on the degree of polymerization as well as upon the nature of the end groups in the polymer. These groups can be relatively inert chemically, e.g., the trimethylsiloxy group, or they can be chemically active. An example of the reactive type is the silanol group.
It has been known for many years now, as earlier indicated, that certain silicones (organosiloxane polymers), or formulations thereof, can be applied to the surface of paper, various films and other substrates to render the surface thereof abhesive (i.e., non-adherent) to sticky and tacky material. Exemplary of this prior art are U.S. Pat. Nos. 2,882,183; 3,050,411; 3,328,482; 3,518,325; 3,565,838; 3,671,484; and 3,823,025. Of these, U.S. Pat. No. 3,518,325 discloses that, optionally, an isocyanate having at least two isocyanate groups per molecule, or a reaction product of such an isocyanate with a polyhydroxy compound, can be included in the silicone composition.
Organo siloxane compounds of the type disclosed in these patents are, in general, liquid materials having mainly dimethylsiloxy repeating units with a few functional groups chemically bonded to the main polymer chain. The liquid polymer is applied in solvent solution to which a catalyst such as dimethyl tin diacetate has been added. On evaporation of the solvent, a surface film of the liquid polymer is deposited which builds up in molecular weight, crosslinks, and a solid coating results. This process conventionally needs either high temperatures or long time at room temperature with the presence of moisture.
The necessity for curing the polymeric material after deposition has placed certain restrictions on the use of various siloxane polymers. This is because the temperature needed for curing, at least to the extent desired, has often exceeded the temperature which a substrate on which a release coating is desired could withstand. This has been particularly true where the substrate is a paper-thermoplastic film laminate, care being needed to avoid melting or otherwise ruining the thermoplastic film.
A further disadvantage associated with silicone release polymers with which I am familiar is their relatively poor adhesion to the various substrates on which they are coated. This poor adhesion is thought to be due to the scarcity of polar groups in the organo siloxane polymer. Consequently, the use of a primer is often needed to obtain better anchorage of the silicone film to the substrate. The fact that silicone polymers have good release characteristics in itself is sometimes a problem in that too good a release is provided. Thus, there may be too easy unwind in tape rolls and, in some cases, the adhesion between the adhesive mass and release coat may be so low that the roll doesn't remain tightly wound.
Heretofore others have disclosed the manufacture of polysiloxane-urethane copolymers. In U.S. Pat. No. 2,511,310, there is disclosed linear polymers from the reaction of bifunctional organo-silicone compounds, e.g., of the formula R.sub.2 Si (OH).sub.2 wherein R is a monovalent hydrocarbon radical, with diisocyanates and diisothiocyanates, the reaction being preferably conducted at temperatures of 35.degree. to 200.degree. C. in an inert, anhydrous solvent. The polymers are, in general, solids of high softening points, e.g., 195.degree. C and higher, and are soluble in, for example, phenol, and in some cases, formic acid and benzene. The polymers disclosed in this patent are said to be useful in the preparation of fibers, films, and plastics.
There has also been developed in recent years organofunctional silicone fluids, e.g. silicone polycarbinols, which combine typical silicone properties, for example, release, with alcohol reactivity. The silicone polycarbinols can be chemically bonded into any system which is reactive toward alcohol to permanently impart desirable silicone properties to that system. Thus, there can be provided silicone modified polyesters, silicone modified sulfonate, silicone modified urethanes, silicone modified melamines, and silicone modified phenolics.
Another organofunctional silicone fluid developed is one containing mercapto end groups. These mercaptosilicone compounds are disclosed in U.S. Pat. Nos. 3,388,144 and 3,388,145 assigned to Dow Corning Corporation. As disclosed in the latter patent (Example 1) a mecaptopolysilicone having a visosity of about 50-60 cs at 25.degree. C can be used as a mold release agent for synthetic rubber. Compounds of the type disclosed in 3,388,145 can also be chemically bonded to rubber to give anti-blocking characteristics to uncured rubber stock thus obviating the need for, e.g., talc.
In U.S. Pat. No. 3,725,000 (Dow) there is disclosed a polysilicone lubricating finish for Spandex fibers in which a radical is provided on the polysiloxane containing an isocyanate reactive group, e.g. isocyanate, amino, hydroxyl, and mercapto groups, reactive with the Spandex fibers. Thus, when the polysiloxane finish is applied to the fibers, apparently a chemical bond results between the two which is believed responsible for the non-migrating characteristics of the lubricating finish.
Other prior art of which applicant is aware are U.S. Pat. Nos. 3,170,891 and 3,509,193. The former patent is directed to an organosilicone isocyanate. As disclosed therein, the isocyanate radical of such a compound will react with any active hydrogen, such as a hydrogen attached to oxygen, nitrogen, and sulfur. The invention in the latter patent is directed to the manufacture of a mercaptomethyl substituted organopolysiloxane useful as a water-repellant impregnating agent.
Moreover, it has been suggested heretofore to use a silicone release agent in combination with a film forming resin, e.g. melamine or urea formaldehyde resin. Such a mixture is disclosed in U.S. Pat. No. 3,061,567. Thus, it has been found with such a composition that the organic resin is apparently preferentially absorbed on the substrate surface on which the release composition is deposited. This allows the silicone to be concentrated on the surface, thereby making for better release. For a given release less silicone is needed. The ratio of organosiloxane to organic resin in the mixture, as disclosed by the patentee, is not critical and can vary over a wide range.